Laser pump module with reduced tracking error

ABSTRACT

A laser package comprises a laser diode source having a first Fabry-Perot cavity between a highly reflective back facet and low reflective front facet for providing a first light output for an optical application. A light monitor is positioned adjacent to the back facet and aligned to receive a second light output from the laser diode back facet. A pigtail fiber having a lensed fiber input end is positioned from the laser diode front facet to form an optical coupling region and is aligned relative to the lasing cavity to receive the first light output into the fiber, the light output exiting the package for coupling to the application. A portion of the first light output from the lasing cavity is reflected off the lensed fiber input end with a portion directed back into the lasing cavity and another portion reflected off of the laser diode front facet. The front facet forms with the lensed fiber input end a second Fabry-Perot cavity generating light which is periodically in and out of phase with the light generated in the first Fabry-Perot cavity due to changes in the length of the second Fabry-Perot cavity caused by package ambient temperature changes so that a tracking error is generated in a signal developed by the light monitor. Thus, this invention provides several ways to suppress the formation of the second Fabry-Perot cavity.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.09/915,184 entitled TRACKING ERROR REDUCTION AND METHOD OF REDUCINGTRACKING ERROR by Wolak et al., filed Jul. 24, 2001, the disclosure ofwhich is incorporated for all purposes. This application also claimspriority from U.S. Provisional Patent Application, serial No. 60/287,936entitled TRACKING ERROR SUPPRESSION AND METHOD OF REDUCING TRACKINGERROR by Edmund Wolak, Tae Jin Kim, and Harrision Ransom, filed Apr. 30,2001; and from U.S. Provisional Patent Application, serial No.60/291,949 entitled ADVANCED LENS SHAPES FOR FIBER COUPLED LASERS WITHREDUCED FEEDBACK by Edmund L. Wolak, Lei Xu, and Robert Lang, filed May21, 2001; the disclosures of which are hereby incorporated in theirentirety for all purposes. This application is being concurrently filedwith U.S. patent application Ser. No. 09/915,186, entitled LENSEDOPTICAL FIBER by Edmund L. Wolak, Li Xu, Robert Lang and Tae J. Kim, thedisclosure of which is hereby incorporated for all purposes.

FIELD OF THE INVENTION

This invention relates generally to monitoring of light sources and moreparticularly to the suppression of tracking error in the monitoring ofthe output intensity of laser source, such as in the case of a laserpump module. However, this invention is equally applicable to any otherapplications where tracking of the intensity or power output of a lasersource is required, albeit a semiconductor laser, a fiber laser or asolid-state laser.

BACKGROUND OF THE INVENTION

In the employment of pump laser modules, such as 980 nm and 1480 nm pumpmodules, for optical telecommunication applications, it is necessary toinsure that the output intensity of the pump laser is maintained at adesired level. This is currently done by monitoring the output powerthat is provided out of the pump module pigtail fiber using a monitordevice, such as a monitor photo diode (MPD), positioned at the backfacet of the laser diode chip in the module package. The pump moduletypically comprises a laser diode chip with its front facet light outputprovided from the laser cavity aligned to be optically coupled into asingle mode pigtail fiber which fiber terminates externally of thepackage for splicing to a fiber amplifier or fiber laser or other typeof optical application. The optical coupling of the laser diode outputhas been accomplished by means of a lens that collimates and focuses theoutput light into the input end of the fiber. Some of the laser diodeoutput light is reflected back from the lens back into the laser cavity,where it is amplified in the laser cavity and exits, in part, out of theback facet to the MPD. Another portion of the output light is scatteredand lost within the module case or package. Light reflecting from thelens or other optical element may be detected directly without the lightpassing thorough the diode waveguide.

A more attractive approach for coupling this light is the use of apigtail fiber that has a lens formed on its input end such as chisel orwedged shaped lens, as disclosed in U.S. Pat. Nos. 5,940,557 by Harker;5,455,879 by Modavis et al.; 5,500,911 by Roff, and 5,074,682 by Uno etal.; all of which are incorporated herein by their reference. Inparticular, if the chisel shaped input end of the pigtail fiber isangled relative to the longitudinal axis of the fiber, furtherimprovements in coupling efficiency can be realized as set forth in U.S.Pat. No. 5,940,557. The angled lens with an anti-reflecting (AR) coatingplaced on its surface prevents a significant portion of laser diodeoutput light reflected off the input chisel lens from reentering thelaser diode chip.

As is well known in the art of laser diodes, the back facet of the pumpmodule laser diode has a high reflecting (HR) coating while the frontfacet has a low reflecting or anti-reflecting (AR) coating so that mostof the laser diode optical power in the laser cavity will emanate fromthe front facet while being highly reflected at the back facet. However,a HR reflector is not a perfect reflector so that approximately 0.5% to10% of the laser light will penetrate the HR coating and can be employedwith the MPD to track the output power of the laser diode by sensing theback facet light from the laser diode. Another way of checking andmonitoring the output power is split off a small portion of the outputpower, e.g. 0.5% or 1% and feed this small amount to an MPD. Typically,the monitor current is going to be about 0.5 to 1 milliamp of currentper milliwatt of power from the laser diode chip back facet impinging onthe MPD. It has been traditionally preferred to place the MPD at theback facet of the laser diode to take advantage of the small amountpower emanating from the back facet of the diode.

One problem with the MPD detector in the package is that with changes inthe ambient temperature within the module package for a given outputpower from the module, the MPD changes in value with such temperaturechanges. In use of the pump module, end users desire that, for a givenMPD current output, a given optical output power can be derived from themodule. However, there is always some variation to be expected withchanges in the case temperature, but it is required to be withintolerable limits or range, which is now considered between about ±5-10%with a package temperature variation from about 0-75° C. In other words,a tracking error of MPD with ±8% is presently acceptable but valuesbeyond this range are not generally acceptable to end users. Also, themaximum acceptable tracking error will likely be required to be reducedas end user's demands for higher accuracy continually increase, imposingfurther suppression of tracking error by pump module manufacturers.Tracking error herein is defined as the change in module output powerwith the change in case or package temperature for a fixed MPD currentdeveloped from the light output collected from the laser diode backfacet by the MPD. We have experienced back facet MPD tracking errors inexcess of this range and, therefore, something needs to be done toprovide for more accurate tracking of the output power of the module tomeet the need of end users.

There are several complicated factors in determining the cause oftracking error but two of the principal causes are described as follows.As the module case temperature changes with operation or with ambienttemperature, the inside ambient of the pump module package, where thelaser diode chip and MPD are positioned, is set to be at a predeterminedoperating temperature using a thermoelectric cooler (“TEC”), which maybe any number of different operating temperatures but is typically 25°C. This is done so that the operating temperature remains the same sothe optical characteristics of module operation do not significantlychange with ambient temperature.

However, as the module package temperature changes during operation, thepackage, and particularly the platform supporting the laser diode andthe coupling pigtail fiber input end, will flex or warp ever so slightlycausing slight internal misalignment between the lensed fiber input tipor end and the laser front facet. This distance or cavity length betweenthe fiber lens and the laser diode front facet is typically around 10μm. Compared to the cavity length of the laser diode chip, this is quitesmall. The typical cavity length of a 98 nm chip is about 1.5 mm and thecavity length of a 1480 chip is about 2 mm.

The relative reflective feedback off the lensed fiber tip and thereflected light off of the external surface of the laser front facetform a Fabry-Perot (F-P) cavity. Thus, there are two such F-P cavitiesexisting in the package—the laser Fabry-Perot (F-P) primary cavity andthe facet-to-lens Fabry-Perot (F-P) secondary cavity wherein reflectedlight from these component surfaces in the secondary cavity achievessome degree of resonance. As the case temperature changes, the distancebetween the laser front facet and the fiber lens tip can change by asmall amount.

Changes in the length of the secondary F-P cavity arising from changesin the case temperature causes the light in this secondary cavity to gointo and out of phase with the phase of the light generated in the laserdiode chip, adding to and subtracting from the light emitted from thelaser diode. This change in phase does not have much effect on the pumpmodule output power because the light reflected between the front facetof the laser diode and the face of the lensed fiber is relatively smallcompared to the total light output from the laser diode. However, thesechanges in phase interference can have a significant effect on the MPDbecause the feedback going into the laser diode from the secondary F-Pcavity is amplified in the laser diode chip and the amplified output isdetected by the MPD. Thus, the MPD detects a value that is not trulyrepresentative of the output intensity of the laser diode and the valuedetected by the MPD changes with the phase interference between theprimary and secondary cavities even though the output power from themodule changes very little, if at all.

Another effect on MPD tracking error is fiber lay or positioning in thepackage, which, due to changes in the birefringence, can change theeffective grating strength in the fiber Bragg grating in the pigtailfiber. Changes in case temperature can cause changes in stress on thefiber, particularly in the module snout. Changes in such stress causechanges in the fiber birefringence, which in turn can cause variedamounts of circular polarization. Light reflected off the fiber gratingand fed back into the chip will only amplify in one polarization of thelight. Thus, changes in stress in the snout with temperature can causechanges in MPD current.

In view of the foregoing, tracking error can be caused by the flexing ofthe package platform supporting the laser diode chip and the input endof the pigtail fiber, as well as feedback light entering into the laserdiode cavity where it is amplified and detected by the MPD in additionto other light emitted from the laser cavity emitted from the backfacet.

OBJECTS OF THE INVENTION

Therefore, it is an object of the present invention to overcome theaforementioned problems.

It is a further object to suppress or otherwise reduce tracking error inpump module output power monitoring to acceptable levels.

SUMMARY OF THE INVENTION

According to this invention, several solutions are provided forsuppressing monitoring tracking error.

One solution for reducing tracking error is to employ a biconic lens,instead of a chisel lens, on the input fiber tip in order to suppressthe interference caused by the light reflection feedback from the lenson the fiber tip. Additionally, the results are further improved withthe biconic lens being angled a few degrees on the input fiber tiprelative to the longitudinal axis of the fiber. The angled biconic lensis then spatially positioned from the laser diode output emission withthe axis of the laser cavity and, in addition, may be also angularlydisposed relative to the longitudinal axis of the pigtail fiber. Thishas been shown to reduce the change in MPD output current with fixedlaser diode power output. The biconic lens has a continuous curvedsurface whereas the use of a chisel lens has some locally flat surfacesproviding stronger feedback reflection. With the use of a biconic lensedfiber input end, there is less feedback of reflected light back into thelaser diode cavity. Also, an AR coating may be added to the lens surfaceto reduce its reflection.

Another solution is to employ a biconic lens whose center is offset fromthe center of the fiber core by a few microns rather then employing anangularly disposed biconic lens on the fiber. In the case here, thecenter of the biconic lens radius in the plane of the laser diodejunction is laterally offset from the center axis of the fiber inputend. As a result, light reflected off of the end of the offset biconiclens would tend to be reflected at an angle to the axis of the lasercavity and, therefore, not fed back into the laser diode cavity. Suchoffset reflected light would avoid establishing the F-P secondary cavitythat leads to tracking error.

Another solution is employing a chisel lens so that a substantialportion of the reflected light from the chisel lens would not reflectback into the laser diode cavity, especially with the additionalcompound angle placing the axis of laser diode chip at an angle withrespect to the axis of the fiber. Angling a chisel lens with respect tothe axis of the optical fiber can reduce tracking error at the monitor,e.g. the MPD by avoiding the formation of a strong secondary F-P cavitybetween the front facet of the laser diode and the chisel lens.

A further solution for reducing tracking error is to strengthen therelatively low coefficient of thermal expansion platform supporting thelaser diode source and the coupling fiber supported on the TEC in thepackage. Preferred materials for such submounts are those with highthermal conductivity. Such materials include ceramics and AlN. Byrendering the platform thicker without exceeding the physical limits ofthe package, the tendency for flexing movement between the laser diodefront facet and the lensed input tip or end of the pigtail fiber will besubstantially mitigated. This solution in combination with any of theother solutions described and disclosed herein provides for an enhancedsuppression of monitor tracking error.

Another solution for reducing this tracking error is to move the MPD toanother location in the package rather than at a position at the backfacet of the laser diode chip. One such location is adjacent to thecoupling region between the laser diode front facet and the lensed inputfiber tip where it can detect light lost from the light output from thelaser diode front facet. About 30% of the laser light output istypically lost internally in the package due to light divergence andscattering. By detecting light from the front region of the laser diodechip, the laser chip no longer functions as an amplifier of backwardreflected light into the laser cavity which magnifies the effects ofsmall changes in the effective front facet reflectivity or changes inthe F-P secondary cavity reflectivity. In one embodiment the MPD is tothe side of the coupling region and in another embodiment the MPD isbeneath the coupling region. Another location is to the side where theMPD monitors reflections off the lensed fiber end.

A still further solution is to increase the reflectivity strength of afiber Bragg grating formed in the pigtail fiber for feedback of aportion of the light for wavelength stabilization of the laser diode. Ifthe fiber Bragg grating reflectivity level is significantly greater thanthe reflectivity level of the front facet experienced by the laserdiode, small changes in the effective front facet reflectivity orchanges in the secondary cavity will have a diminished effect on thechanges in the light level emitted from the back facet of the laserdiode. Typically the grating reflectivity level may be anywhere between0.3% to 3% of the transmitted light in the fiber and, further, may beless than the reflectivity of the laser diode front facet.

By increasing its reflectivity level, for example to 6%, changes inpackage temperature affecting the secondary cavity length or effects inthe amount of reflectivity from the lens fiber tip or front facet backinto the laser cavity become insignificant due to the comparatively highamount of feedback light from the grating to stabilize the laser diodeoperation. The use of two or more gratings can reduce the effectsarising from birefringence changes in the snout.

Another solution is to coat the end of the fiber lens so as to be morereflective than the reflectivity level of the output front facet or tocoat the diode facet so that its reflectivity at the peak wavelength,such as 980 nm, is significantly higher than that off of the surface ofthe fiber tipped lens, in either case suppressing the establishment ofthe a F-P secondary cavity. This is because F-P cavities exhibitstronger characteristics if the opposed reflecting surfaces establishingthe cavity have similar reflective levels.

Other objects and attainments together with a fuller understanding ofthe invention will become apparent and appreciated by referring to thefollowing description and claims taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference symbols refer to like parts.

FIG. 1A is a perspective view of a biconic lensed fiber end or tip inaccordance with one aspect of this invention.

FIG. 1B is a side view of the biconic lensed fiber end shown in FIG. 1A.

FIG. 1C is a plan view of the biconic lensed fiber end shown in FIG. 1A.

FIG. 2A is a plan view of a lensed fiber end in conjunction with a laserdiode where the center axis of the lens is fractionally offset from thecenter axis of the optical fiber.

FIG. 2B is a plan view of an angled chisel-shaped lensed fiber end ofthe type disclosed in U.S. Pat. No. 5,940,577.

FIG. 2C is a simplified top view of a lensed optical fiber according toan embodiment of the present invention illustrating an offset center ofthe radius of the lens.

FIG. 2D is a simplified side view of the lensed fiber illustrated inFIG. 2C showing the core of the optical fiber centered with respect tothe curve of the lens.

FIG. 3A is a plan view of an angled chisel or wedged shaped lensed fiberend where a side portion of the lens is cut away for angular positioningrelative to the laser diode chip front facet.

FIG. 3B is a side view of the angled wedged-shape lensed fiber end shownin FIG. 3A.

FIG. 3C is an input end view of the angled wedged-shape lensed fiber endshown in FIG. 3A.

FIG. 4A is a perspective view of a biconic lensed fiber according toanother embodiment of the present invention.

FIG. 4B is a plan view of a biconic lensed fiber in relation to a laserdiode.

FIG. 4C is a side view of an enlarged portion of the biconic lensedfiber end shown in FIG. 4B.

FIG. 4D is an end view of the biconic lensed fiber end of FIG. 4Aillustrating an offset in the center of curvature of the biconic lensfrom the optical axis of the fiber.

FIG. 5A is a simplified top view of a lensed optical fiber according toan embodiment of the present invention.

FIG. 5B is a simplified cross section of the lensed optical fiber ofFIG. 5A illustrating a pointed chisel lens.

FIG. 5C is an enlarged portion of the cross section illustrated in FIG.5B.

FIG. 5D is a diagram illustrating a pointed chisel lens with doublyoffset radii.

FIG. 5E is a simplified cross section of a pointed chisel lens with twodifferent radii.

FIG. 5F is a simplified top view of a lensed optical fiber with a doublechisel lens, according to another embodiment of the present invention.

FIG. 5G is a simplified end view of the lensed optical fiber shown inFIG. 5F.

FIG. 5H is a simplified cross section of the lensed optical fiber shownin FIG. 5F.

FIG. 5I is a simplified top view of another lensed optical fiber with adouble chisel lens having coupling surfaces at two different angles fromthe center axis of the optical fiber.

FIG. 5J is a simplified top view of a lensed optical fiber with apointed chisel lens according to another embodiment of the presentinvention.

FIG. 5K is a first cross section of the lensed optical fiber shown inFIG. 5J.

FIG. 5L is a second cross section of the lensed optical fiber shown inFIG.

FIG. 5M is a simplified front view of the lensed optical fiber shown inFIG. 5J.

FIG. 5N is a simplified cross section of a doubly lensed optical fiberin the slow direction.

FIG. 5O is a simplified cross section of the doubly lensed optical fiberof FIG. 5N in the fast direction.

FIG. 5P is a simplified view of a lensed optical fiber with a pointedlens according to another embodiment of the present invention.

FIG. 5Q is a simplified cross section of the lensed optical fiber shownin FIG. 5P.

FIG. 5R is an enlarged portion of the cross section shown in FIG. 5Qillustrating facets forming a point on the end of the lensed opticalfiber.

FIG. 5S is a simplified top view of an optical fiber with an integratedFresnel-type lens.

FIG. 5T is a simplified cross section of the lensed fiber shown in FIG.5S.

FIG. 5U is a front view of a lensed fiber with a Fresnel-type lens.

FIG. 5V is a simplified cross section of the lensed fiber of FIG. 5U.

FIG. 5W is another simplified cross section of the lensed fiber of FIG.5U.

FIG. 5X is a simplified cross section illustrating a binary Fresnel-typeoptical fiber micro-lens.

FIG. 6A is a simplified cross section of an angled biconic lensaccording to another embodiment of the present invention.

FIG. 6B is a simplified cross section in another plane of the angledbiconic lens shown in FIG. 6A.

FIG. 6C is a simplified end view of the angled biconic lens shown inFIGS. 6A and 6B.

FIG. 7 is a plan view of a pump module arrangement of a light monitor,laser diode and lensed fiber input end with a fiber lens of the typeshown in FIG. 7A.

FIG. 8A is a perspective view of a pump module arrangement shown in FIG.6 on a module platform without the module package.

FIG. 8B is a simplified side view of the pump module shown in FIG. 7A.

FIG. 8C is a simplified side view of a pump module according to anotherembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference is now made to FIG. 1A wherein there is shown a lensed fiberend 10 or tip comprising a biconic lens 12. The lens is formed on thefiber using special processing steps to form the curvatures of the lenssurfaces and has a shape similar to a weather pyramid. The biconic lens12 has curvatures that are different in orthogonal directions asdepicted in FIGS. 1B and 1C. In one orthogonal direction, as shown inFIG. 1B, a first radius of curvature 11 in is 14 μm whereas in the otherorthogonal direction a second radius of curvature 13 is 8 μm, with atapered angle Θ₁ of about 50° to 55°. Such a lens is also shown inconcurrently-filed co-pending U.S. patent application Ser. No.09/915,186, entitled LENSED OPTICAL FIBER by Edmund L. Wolak, Lei Xu,Robert Lang, and Tae J. Kim which is assigned to the assignee herein andis incorporated herein by its reference. The larger radius in the planeof the lens may be, for example, around 12-22 μm while in the sideelevation orthogonal to this plane the radius of curvature may be, forexample, around 5-10 μm. As set forth in application Ser. No.09/915,186, the biconic lens provides for improved coupling efficiencycompared to a chisel or wedged-shaped fiber lens. The use of a biconiclens 12 has shown to reduce the change in the laser diode monitor outputof laser monitor 15 (FIG. 2A), for example, a monitor photo diode (MPD),due to a difference in the level of reflected light feedback employing abiconic lens over a chisel or wedged-shaped lens. Monitor photo diodescan be avalanche diodes and PIN photodiodes, among others. The biconiclens has a continuous curved surface whereas the use of a chisel lenshas some locally nearly flat surfaces that can provide some feedbackreflection. With the use of a biconic lensed fiber input end, there isless feedback of reflected light back into the laser diode cavity. Also,an AR coating is preferably applied to the biconic lens surface toreduce its reflection capabilities in the range of wavelengths producedin the laser diode output.

As shown in FIG. 2A, the biconic fiber lensed input end 10 of pigtailfiber 14 is positioned in front of the laser diode 16 to receive thelight output from the diode via front facet 17. Laser diode 16 alsoincludes a back facet 19, which has a highly reflective (HR) coating onit surface to reflect around 93-98% of the laser cavity light back intothe laser cavity 21. However, 2% to 7% of the light penetrates throughthe back facet 19 and is received by laser monitor 15 to produce acurrent signal indicative of the laser diode output power or intensity.

In the case of the embodiment of FIG. 2A, the center of curvature of thebiconic lens lies along a line 20 that is offset from the center axis18′ of the fiber, which in this embodiment is aligned with the cavityaxis 18 of laser diode 16. In other embodiments the center axis of thefiber is angled a few degrees from the cavity axis of the laser diode.In this way, a majority of the light output from the front facet 17 iscaptured and collected by biconic lens 12 but any reflected light off ofthe surface of lens 12 and propagating back into the laser cavity 21 oflaser diode 16 is minimized. The amount of offset is dependent on thedistance between the laser diode front facet 17 and the shape of biconiclens 12 as well as the size of the single mode core of fiber 14, but itmay be in the range, for example, of several microns. As previouslymentioned, an AR coating is preferably applied to the surface of biconiclens 12 in some embodiments to reduce its reflection capabilities.

Reference is now made to FIG. 2B, which illustrates an angled chiseltype lens 32, such as is discussed in U.S. Pat. No. 5,940,557,incorporated herein, on a fiber end 30. As shown in FIG. 2B, the axis oflens 32 is angled with respect to the normal of the center axis 34 offiber end 30. In the example here, the angle Θ₂ from the normal to thefiber longitudinal or optical axis 34 may be around 8°, which isexaggerated in FIG. 2B for purposes of illustration. The chisel lensedfiber input end 30 is shown in FIG. 2B with its central axis 34 alignedwith the axis 18 of laser cavity 21. However, as shown in U.S. Pat. No.5,940,557, the axis 18 of laser diode 16 may be aligned at an anglerelative to the axis 34 of the fiber end 30. As previously mentioned, anAR coating is preferably applied to the surface of biconic lens 32 toreduce its reflection capabilities.

The angled chisel lens 32 is used in combination with the laser diode 16and laser monitor 15 to reduce tracking error. In particular, the angledchisel lens reduces the light from the aperture 18 of the front facet 17reflected back into the laser cavity 21 that would be amplified andtransmitted through the back facet 19 of the laser diode to the lasermonitor 15. This amplified back reflected light can cause tracking errorin the system because the light detected by the laser monitor does notaccurately indicate the light output of the laser diode.

FIG. 2C is a simplified top view of an optical fiber 14 according to anembodiment of the present invention illustrating a center 22 of theradius R of the lens 28 and how it is offset from the optical axis 34 ofthe fiber when viewed from the top. The optical axis 34 is essentiallythe center of the core 24 of the fiber 14. The center of the radius isoffset about 2 microns from the optical axis of the core in a particularembodiment for a particular single-mode fiber. Other types of fibersmight have different offsets. Thus, the center of the curved surface ofthe fiber end is also offset from the optical axis of the fiber, asviewed from the top. For purposes of discussion, this type of lens iscalled an offset biconic lens.

When aligning a fiber with an offset biconic lens to the output of alaser diode, the center axis of the fiber core can be aligned with theaxis of the laser diode, without having to angle the fiber with respectto the laser diode output to avoid back reflections. In anotherembodiment, the laser diode is angled with respect to the center axis ofthe fiber, by about 2-15 degrees or about 2-5 degrees, which can improvecoupling and further reduce back reflections. FIG. 2D is a simplifiedside view of the lensed fiber illustrated in FIG. 2C showing that thecurve of the lens 28 is essentially centered with respect to the core 24and center line 34 of the optical fiber 14 in this view, i.e. the centerof curvature for the biconic lens in this section lies on the centerline34.

Another embodiment of an angled chisel lens is shown in FIGS. 3A-3C. InFIG. 3A, angled chisel lens 32A has an angled lens tip 35 such as around8° from the normal to the longitudinal or optical axis of the fiberinput end 30. The radius of curvature of lens tip 35 may be about 8 μmwith a tapered angle Θ1′ in the range of about 50° to 55°. Also, a partof the lens is shaved away at 33 so as to allow close positioning of thelens tip to laser diode 16, such as shown in FIG. 5. The distancebetween laser diode 16 and lensed fiber 14 is very small, such as, forexample 10 μm. By angularly shaving off the lens at 33, the lens 32A canbe positioned very close at an angle relative to laser diode facet 17without contact of the facet by the lens. FIG. 3C is an end view of thelens 32A showing the lens tip 35 and the shaved portion 33, in additionto the faces of the lens.

FIG. 4A is a simplified perspective view of an offset biconic lens 12′on a fiber end 10′ according to another embodiment of the presentinvention. FIGS. 4B and 4C illustrate that the biconic lens 12′ has beenformed so that the center of curvature 36 of the lens lies on a line 20offset from the center 34 of the core of the fiber. In one embodiment,the origin is offset from the center of the fiber by about ¼-⅔ of thecore diameter. In a particular embodiment, the origin is offset about 2microns from the center axis of the fiber.

FIG. 4B is a plan view illustrating that the axis 18 of laser diode isnominally aligned to intersect the center 34 of the core of the fiber atthe fiber end, with the axis of the laser diode being at a slight anglefrom the center axis of the fiber. In a particular embodiment, the anglebetween the axis of the laser source cavity and the center of the coreis between about 2-6 degrees. The drawing is not to scale in order toillustrate the fiber end more clearly. The center of curvature 36 of thelens 12 lies along a line 20′ offset from the center of the fiber core34. FIG. 4C is a top view of the fiber end 10′ and the biconic lens 12′illustrating the offset of the center of the lens' surface from thecenter axis 20′ of the core of the fiber. In a particular embodiment,the curvature in the top view elevation axis is picked to match thedivergence of the light coming out of the laser. This lens is ananamorphic lens, i.e. it provides different powers at different axis ofthe lens.

FIG. 4D is an end view of the fiber end 10′ illustrating the core of thefiber with a circle 39, and the center of curvature 36 of the biconiclens. In a particular embodiment, the core has a diameter of about 6microns and the center of curvature of the biconic lens is offset fromthe center of the core about 2 microns with a 15 micron radius for theslow arc and an 8 micron radius for the fast arc. Other dimensions mightbe appropriate for other types of fibers. Generally speaking, offsets inthe range of ⅓ to ⅔ of the mode field diameter are preferred. Offsettingcan be done in the slow axis, if desired.

FIG. 5A is a simplified top view of another lens 62 on a fiber end thatprovides high coupling efficiency and low feedback when used to couplelight from a laser diode. This lens 62 is a chisel lens that isnominally symmetrical about the optical axis (i.e. center of the core)of the optical fiber 14 and has an edge 63 that comes to a point. FIG.5B is a section along A of the lens illustrated in FIG. 5A. The edge ofthe lens comes to a point 64 that is fabricated by lapping the fiber endat a radius offset from the center axis of the fiber on each side 66, 68of the chisel lens. Although the edge of the chisel lens is shown as astraight segment in FIG. 2A, the edge could be curved, angled, orsharpened, as discussed above in relation to FIGS. 3A-3C and below inrelation to FIGS. 5F-5H, FIGS. 5J-5L, and FIGS. 5N-5R.

FIG. 5C is an enlarged view of the point 64. Lapping at offset radiiavoids the formation of a “flat” spot at the end of the fiber. Althoughlapping on a radius that lies along the center axis, as isconventionally done, produces a lensed fiber end with a very smallradius of curvature, even such a small curvature can provide a surfacethat looks relatively flat to a light beam. This flatish surface canreflect light back into a laser diode, for example, while the pointproduced according to this embodiment provides much less reflection,even though the end of the lensed fiber is not a perfect point, that isto say, some softening of the point typically occurs due to thefabrication techniques. Other fabrication techniques, such as laserablation or diamond turning, might be used to fabricate a pointed chisellens.

FIG. 5D is a simplified diagram illustrating the configuration of apointed chisel lens 70. The centers of curvature 72, 74 are each offsetfrom the centerline 76. Alternatively, only one center of curvature isoffset from the centerline. The tangent lines 78, 80 form an angle α ofbetween about 176-156 degrees, which is exaggerated in this view forpurposes of illustration.

FIG. 5E is a simplified cross section of a pointed chisel lens 82 withoffset lapping radii of different lengths. This produces lens surfaces84, 86 with different curves, but the lens still comes to a point 64′.

FIG. 5F is a simplified top view of a double chisel lens 88. The term“double chisel” means that there are two angled chisel structures 69, 71formed on the fiber end. Both chisel structures are angled with respectto a plane orthogonal to the center axis of the fiber and intersect toform a point 73. While the lens edge of the pointed chisel lensillustrated in FIG. 5A is pointed, the end of the double chisel lenscomes to a “sharpened” point. In an alternate embodiment, one chisel isnot angled and the other is. The point is preferably within or very nearthe core, and may be offset from the center of the core. The doublechisel structure provides improved alignment tolerance because thecutaway portions of the fiber end avoid mechanical interference with thefront facet of the source. The terms “vertical” and “horizontal” areused for purposes of convenient discussion only and generally relate tothe orientation of the fiber when mounted to a substrate, with the majorplane of the substrate being essentially parallel to the horizontalaxis. Other terms, such as “slow” and “fast” axis are also often used todescribe the relative orientations.

The angle β between a plane orthogonal to and the lens edge 91 formed bythe chisel faces (see FIG. 5G, ref. nums. 87, 89) of the chiselstructure 71. Having an angle greater than 2 degrees provides goodfeedback suppression, which an angle less than 12 degrees maintains goodoptical coupling to a laser diode.

It is believed that the point 73 formed by the intersection of the twochisel structures 69, 71 provides some additional lensing action whilereducing the flat area that might reflect light back into the source.The lens edges 91, 91′ typically have a radius of 5-11 microns in asingle-mode silica-based fiber, but could have a lesser or greaterradius. In one embodiment, the radius of the first lens edge 91 isessentially the same as the radius of the second lens edge 91′. Inanother embodiment these radii are different.

The chisel faces 87, 89 of the first chisel structure 71 intersect thecorresponding faces 87′, 89′ of the second chisel structure 69 to formthe vertical ridge 75. In a particular embodiment, the vertical ridgecomes to a relatively sharp edge, typically with a radius of about 2microns or less, but could be intentionally or incidentally rounded witha greater radius. The vertical ridges 75, 75′ form a sharpened point 73with the lens edges 91, 91′.

FIG. 5G is an end view of the double-chisel lensed fiber end 88 shown inFIG. 5E showing the lens edge 91 formed by the chisel faces 87, 89 ofthe first chisel structure 71 and the vertical ridges 75, 75′ formingthe point 73. FIG. 5H is a section taken along the section line Bshowing the radiused nature of the lens edge 91 formed by chisel faces87, 89 at the point 73 when viewed from this orientation. A section ofthe second lens edge (see ref. num. 91′, FIGS. 5F, 5G) would besimilarly radiused. Alternatively, the lens edges could be pointed, inaccordance with FIGS. 5A-5E, above.

FIG. 5I is a top view of an alternative embodiment of a double chisellens 88′ on a fiber end with lens edges 90, 100 each at a differentangle β₁, β₂ from a plane orthogonal to the center axis of the opticalfiber. Both angles are between about 3-12 degrees, and the differencebetween angles (i.e. β₁−β₂) is between 1-3 degrees. This accommodatesmisorientation errors when fabricating the laser diode source assembly.For example, the laser diode might be misoriented 2 degrees from itsdesired orientation to the fiber when the laser diode is attached to thesubmount. The fiber can then be aligned with first one angle (e.g. α) ona side and then the other angle (β) on the side by rotating the fiber180 degrees in the assembly tooling. The orientation that provides thebest coupling to the laser diode can be selected and the fiber fixed inthis position.

Lensed fibers are often mounted at an angle to a laser diode source andin close proximity, about 8-10 microns away from the emitting facet insome cases. As described in the preceding paragraph, one face of thelens can be oriented toward the laser diode for optimized coupling. Theother face of the lens serves as cut-away relief so that the fiber endcan be mounted close to the laser diode without physical interferencebetween the components.

FIG. 5J is a simplified top view of a lensed fiber 110 with anotherchisel lens 112 according to an embodiment of the present invention. Twosection lines, C, D, are illustrated in FIGS. 5K and 5M, respectively. Afront view is shown in FIG. 5L. This lens might be easier to fabricatethan the similar lenses shown in FIGS. 5F and 5G.

A lensed fiber according to FIG. 5J is made by lapping a conventionalchisel lens arm then grinding two faces 114, 116 to form a point 118 onthe end of the fiber. The cross section of FIG. 5K shows a curved endformed by lapping about a radius along the center line of the fiber, buta pointed end, such as are discussed in relation to FIGS. 5C and 5Ecould also be fabricated. When mounted in an assembly, one of the facesprovides cut-away relief for close assembly to a light source, while theother face forms a lens-like point with the first face to improveoptical coupling in the slow direction.

FIGS. 5N and 5O are simplified cross sections of a pointed double lensedfiber end illustrating different radii in the fast and slow directions.FIG. 5N represents a cross section of the lens structure for coupling inthe slow direction with a radius forming the surface 120 of betweenabout 12-22 microns. The surface of the lens shown in cross section inFIG. 5O comes to a point 122 from a radius of between about 5-11microns. In a particular embodiment, the radius in the slow direction isabout twice the radius in the fast direction. Alternatively, the pointcan be formed in the cross section of the slow direction (see FIG. 5N).The fist and slow directions generally lie along orthogonal axes, andthe doubly lensed fiber end is pointed on at least one of these axes.

Radii can be between about 12-22 microns in the slow direction andbetween about 5-11 microns in the fast direction, but these dimensionsare only examples for an embodiment using a conventional laser diodechip and optical fiber. A suitable value is chosen according to thefar-field emission pattern of the light source that the lensed fiber iscoupling to and other considerations, such as the index of refraction ofthe material that the lens is made of and the diameter of the core ofthe optical fiber, which in one example is about 6 microns.

FIGS. 5P-5R illustrate another way to make a pointed chisel lens fiber.FIG. 5P is a top view of a lensed fiber 130 according to anotherembodiment of the present invention. FIG. 5Q is a cross section alongsection line E and FIG. 5R is an enlarged view of the fiber endillustrating a point 132 formed by cutting facets 134, 136 in theradiused surface 138 of the fiber end. This embodiment of a lensed fibercombines the relatively easy fabrication of a conventional chisel lenswith a pointed lens having reduced reflective feedback into the source(laser diode) waveguide. In one fabrication sequence, first facets 140,142 are ground before lapping the radiused surface 138. Then, the lensis pointed by grinding the second facets 134, 136.

FIG. 5S is a simplified top view of a Fresnel-type chisel lens 150 onthe end of an optical fiber 152. The core of the optical fiber isrepresented by dotted lines 154, 156. This type of lens avoids the lenstip getting too close to the facet of the laser diode when aligned in asource module. The Fresnel-type lens has a series of ridges 158, 160 andcorresponding valleys 162, 164 formed on an edge 166 of the chiselstructure. The ridges and valleys are very fine and at a fine pitch,typically much less than the core diameter, and are not drawn to scale,but are enlarged relative to the fiber for purposes of illustration.

The lens structure is “broken” into these ridges and valleys, which forpurposes of discussion will be referred to a “lenslets”, rather thanangling a chisel lens. This avoids the variation in distance between oneside of the lens structure and the other, compared to a conventionalangled chisel lens. The ridges and valleys are made using laser ablationor diamond turning techniques. In a further embodiment, both the apexesof the ridges and the troughs of the valleys are radiused, with theradius of the troughs being less than the radius of the apexes. In aparticular embodiment the radius of the troughs is about 8 microns andthe radius of the apexes is about 7 microns. Generally, the troughs havea radius of about 1.1-1.4 times the apexes. It is generally desirablethat the radii of each ridge closer to the facet or source are less thanthe radii of the troughs, which are further away. In one embodiment, theradii of all ridges are approximately equal and in other embodiments,the radii are different. For example, if the fiber is angled withrespect to the light source, then it may be desirable to increase theradii of ridges further from the source.

In an alternative embodiment, a Fresnel lens structure is formed on theface of an optical fiber without lapping the fiber end to form a chiselstructure. The Fresnel lens is designed to emulate the opticalcharacteristics of an angled chisel lens.

FIG. 5T is a simplified cross section of the lens shown in FIG. 5S takenalong the section line F showing the radiused edge 166 of the chiselstructure on the end of the fiber 152.

FIG. 5U is a simplified front view of a binary lens 168 according toanother embodiment of the present invention. A series of binary lenslets174 have been formed along the tip of the lens. The shape of the lens issimilar to an elongated truncated pyramid. FIG. 5V is a simplified crosssection taken along section line G and FIG. 5W is a simplified crosssection taken along section line H showing the radiused nature of thetip 172.

FIG. 5X is a simplified cross section of the series of lenslets 170showing how the lenslets 176 are made up of a series of steps 178, 180.These stepped lenslets operate similarly to the lenslets in theFresnel-lensed fibers with straight-faced lenslets shown in FIG. 5S.These binary lenslets may provide easier fabrication than thestraight-faced lenslet.

FIG. 6A is a simplified section of an angled biconic lens 192 on anoptical fiber end 194. Hashing lines are omitted in this sectional viewfor clarity of illustration. Referring to FIG. 6C, this section is takenalong section line I. The angled biconic lens 192 is formed at an angleθ from the centerline 196 of the fiber. This gives the lens a somewhat“bent” appearance from this view in relation to the fiber. FIG. 6B is asimplified section of the fiber end 194 taken along section line J (ref.FIG. 6C), showing that the angled biconic lens can have a “straight”orientation in this view, where the tip of the lens in this section isneither angled to or offset from the center axis 196. While the angledbiconic lens could be angled on both axes, angling on one axis isdesirable to reduce backreflections into a light source, while providinggood coupling efficiency.

The angle θ between the center axis of the fiber 196 and the center axisof the lens 198 is generally between about 2-12 degrees. In oneembodiment, the center axis of the lens intersects the center axis ofthe fiber at the tip 200 of the lens, although in other embodiments thetip of the lens might not be on the centerline of the fiber, but it isgenerally desirable to have the tip within the core portion of thefiber.

FIG. 6C is a simplified front view of the angled biconic lens 192illustrated in FIGS. 6A and 6B, showing the tip of the lens 200 lying onthe center 196 of the fiber end 194. The center of the fiber isgenerally in the center of the core 202. Comparing FIGS. 6C and 4D, thetip of the offset biconic lens illustrated in FIG. 4D is offset from thecenter of the core, while the tip of the angled biconic lens illustratedin FIG. 6C is essentially at the center of the fiber. Generally, thecenter of curvature of the angled biconic lens illustrated in FIG. 6Clies on the line 198, and thus would be offset from the center 196 ofthe fiber.

A common attribute of certain embodiments of a pointed chisel lens, adouble chisel lens, a biconic lens, a Fresnel lens, a binary Fresnellens, an offset biconic lens, and an angled biconic lens is that theycan greatly reduce back reflections into the laser diode source, andthus reduce tracking error in some types of laser modules.

In FIGS. 7, 8A, 8B, and 8C, several different solutions for suppressionof tracking error are illustrated. FIG. 7 is a plan view of a portion ofa laser pump module. As previously explained, the surfaces of laserdiode front facet 17 and lens 32A have some level of reflectivitydespite the use AR coatings on these surfaces so that even with the ARcoating a resonance is experienced by light reflected between thesesurfaces forming a Fabry-Perot secondary cavity 37 in addition to theFabry-Perot primary cavity 16A of the laser itself. Due to changes inambient temperature, the length of cavity 37 can change ever so slightlycausing the light in this secondary cavity 37 to go into and out ofphase with the phase of the light generated in the laser diode 16. Acause of this change in cavity length is flexure or expansion of theplatform 42 due to such temperature changes upon which laser monitor 15,laser diode 16, and lensed fiber input end 30 are mounted as shown inFIGS. 8A and 8B. As previously mentioned, changes in the secondarycavity length can have a significant effect on the laser monitor 15because the net feedback going back into laser diode 16 from thesecondary F-P cavity 37, which also varies in amount over time due tochanges in the length of the secondary cavity 37, is amplified in thelaser diode 16 and the amplified output is detected by the laser monitor15. Thus, the laser monitor 15 detects a value that is not trulyrepresentative of the output intensity of the laser diode 16. One mannerof suppressing the formation of such a secondary F-P cavity 37 is todispose the axial center of lens 32A at an angle to the optical orcavity axis 18 of laser diode 16 as taught in U.S. Pat. No. 5,940,557.Another way is to offset a biconic lens or other anamorphic lens fromthe center of the fiber end. Also, the laser monitor 15 is disposed atan angle with respect to back facet 19 of laser diode 16 so that lightemission 18B from the back facet 19 of the laser diode 16 is notreflected from the laser monitor 15 back into laser diode 16. The lightemission 18A from the front facet 17 of the laser diode 16 also divergessomewhat, but the total divergence of the light illuminating the inputend of the fiber is small compared to the divergence of the lightilluminating the MPD because the lensed fiber end is placed relativelyclose to the front facet of the laser diode, compared to the placementof the MPD from the back facet of the laser diode.

A fiber Bragg grating (“FBG”) 38 has been formed in the fiber 14according to methods that are well known in the art, and illustrated bythe closely spaced bars drawn across the core 14A of the fiber. This FBGreflects a portion of the light from the laser diode back to the laserdiode. The fiber Bragg grating causes said laser diode to operate in thecoherence collapse regime. In one embodiment, the reflectivity of theFBG at the wavelength of the laser diode is about 0.3-3%. In anotherembodiment. The FBG includes two gratings and achieves a reflectivity ofgreater than 6%. The double gratings in the FBG treat circularpolarization light propagating in the package snout such that more lightis reflected back into the plane of polarization of the laser diodesource.

The “snout” is generally a metal tube or cylinder that extends from anoptical module case and provides support for the optical fiber(s)extending from the optical module through the snout. The fiber can becoated with a metal, such as Au—Ni, and soldered in the snout to achievea hermetic seal. The snout often includes a plastic covering or outersleeve, which can extend beyond the metal tube portion of the snout toprovide some strain relief. As an alternative to soldering the fiber inthe snout, a compression fitting can be used to achieve a hermetic seal,which does not require the metal coating as with a soldered seal.

Generally, each fiber coming out of or going into an optical module hasits own snout, and the fibers are commonly called “pigtails”. The end ofsuch a fiber pigtail can be coupled to an optical fiber network byfusion splicing the fiber pigtail to the end of an optical fiber in thenetwork. Another way of optically coupling a fiber pigtail to anotheroptical fiber is to butt-couple the fibers in a capillary in a ferrule.

In a specific embodiment, the reflectivity of the FBG is greater thanthe internal cavity reflectivity of the front facet of the laser diode.Each grating includes a periodic variation of the fiber that reflects aportion of the light transmitted from the laser diode through the fiberback to the laser diode. This reflected light is emitted from the fiberend back toward the laser diode and a small portion of the lightreflected from the FBG is coupled back into the laser diode, providingfeedback at the desired wavelength that can at least partiallycompensate for feedback from a varying secondary FP cavity. The FBG hasa relatively narrow bandwidth, and therefore transmits essentially allof the light that is not at the selected wavelength through the FBGportion of the fiber.

FIG. 8A is a perspective view of a portion of a laser pump module 61.The laser diode 16, laser monitor (e.g. MPD) 15, and input end 30 of thefiber 14 are mounted on a platform 42 that is thicker than housing base44. The fiber 14 is attached to a fiber mount 56 with solder. A portionof the fiber (not shown) covering the soldered section is coated with afew microns of metal to facilitate soldering. The fiber can also beinside a metal tube, such as a KOVAR™ tube, which facilitates welding.An alternative location for an MPD 15′ is also shown in this view. In aparticular embodiment, the fiber or sleeved fiber is held in place witha relatively soft material, such as room-temperature vulcanizing (“RTV”)adhesive or lead-tin solder, compared to an epoxy or hard solder, forexample. Using a soft material to attach the fiber to the fiber mountreduces undesirable effects arising from changes in the package arisingfrom temperature variations.

MPD 15′ is located near the coupling region between the front facet 17of the laser diode 16 and the pigtail fiber end 30, rather than behindthe laser diode, as shown by the conventionally placed MPD 15. Theaperture 21 or emitter of the laser diode is shown for reference. ThisMPD 15′ is illuminated with light from the front facet 17 of the laserdiode 16 and/or light reflected off or emitted from the end 30 of thefiber 14. Locating the MPD near the front, rather than the rear, facetavoids tracking error resulting from amplified back reflections. Therelative positions are not shown to scale, as the fiber end is typicallyvery close to the front facet of the laser diode. The MPD is mounted tothe side of the coupling region (see FIG. 8C, ref. num. 31), which maybe on the order of 10 microns between the source and the end of thefiber. Alternatively, the MPD can be mounted adjacent to the fiber endor the front facet.

If a FBG 38 is included in the fiber 14, light from the laser diode atthe selected wavelength is reflected back not only for laser feedbackpurposes, but some portion of the light reflected from the FBG can becoupled to the MPD 15′ for output level monitoring. Thus, while it isgenerally desirable to transmit light from the laser diode 16 to itspoint of use, providing a FBG, including an FBG having a reflectivity ofgreater than 6%, can be useful when monitoring the output level of alaser diode near the coupling region. The MPD 15′ can be placed tooptimally couple to the light emitted by the source, scattered light, orthe light reflected or emitted by the fiber end, or balanced. Inparticular, the fiber end is usually formed and oriented to avoidbackreflections into the source. A side-mounted MPD 15′ canadvantageously utilize this reflected light to monitor the output of thesource.

Similarly, some embodiments provide an AR coating on the biconic lens32A or other lens formed on the fiber end 30 to reduce reflections thatmight be coupled back into the laser diode. However, the FP cavityformed between the front facet 17 of the laser diode 16 and the end 30of the fiber 14 can be reduced if the reflectivity of the fiber end isincreased above the reflectivity of the front facet of the laser. If arear-mounted MPD 15 is used, this enhanced reflectivity can reduceproblems arising from phasing between the secondary and the primary FPcavities. If a side-mounted MPD 15′ is used, this enhanced reflectivitycan provide a light signal for the MPD to monitor.

The laser diode, fiber mount, and MPDs are generally soldered orotherwise attached to metallized pads 46, 50, 54 that allow movement ofthe associated component on the pad for aligning to the othercomponents. The metallized pads provide secure attachment and, in thecase of active components, also provide bonding pads 48, 52, 60 for wirebonding or otherwise making an electrical connection to the device. Agrounding pad or pads may also be provided on the platform forconvenient connection of the electro-optical devices.

FIG. 8B is a simplified side view of the platform 42 on the housing base44. A thermoelectric cooler (“TEC”) 204 is optionally placed between theplatform 42 and the housing base. The platform can be of the samematerial as the housing base, or of a different material. The platformis shown in this figure as being mounted on the TEC, but the platformcan be mounted directly to the housing base, or the platform and housingbase could be integrated. The platform is preferably made of a materialthat is stiff compared to the material of the housing base. Suchmaterials include silicon, silicon carbide, aluminum nitride, ceramic,such as alumina-based ceramics, sapphire, and diamond. In an alternativeembodiment, the platform is thicker than housing base. In a particularembodiment, the platform is twice as thick as the housing base, and is1.5 mm thick. Choosing a relatively stiff material for the platform, ormaking the platform thicker than the housing base, stiffens the laserpump module between the fiber end and the laser diode, thus decreasingchanges in the secondary cavity length from thermal stresses arisingfrom other areas of the laser pump module, such as the interaction ofother portions of the housing base with the module cover, or can.

FIG. 8C is a simplified side view of a portion of an optical assembly700 according to another embodiment of the present invention. An MPD 715is mounted “under” the coupling region 31 between the front facet 17 ofthe laser diode source 16 and the end 30 of the fiber 14. This mountingis a variation of the “side” mounting discussed in relation to the MPD15′ in FIG. 8A but is oriented to couple to light along a differentaxis. In this orientation, the MPD 715 can take advantage of the widerdispersion of light from the aperture 21, which is typically rectangularwith the long dimension lying in the horizontal plane. Some dispersionalso occurs to the sides of the aperture, as discussed above in relationto FIG. 8A, but the relatively thin and wide waveguide structure of thelaser diode source makes it particularly desirable to place the MPDabove or below or above the aperture because of the available light. Theoptical components are mounted on a substrate 44′ with an integratedplatform 42′ to provide additional stiffness in this region; however,the location of the MPD near the coupling region 31 avoids trackingerror arising from amplified back reflections being emitted from therear facet of the laser diode source. Therefore, other embodiment mightdispense with the platform.

While the invention has been described in conjunction with severalspecific embodiments, it is evident to those skilled in the art thatmany further alternatives, modifications and variations will be apparentin light of the foregoing description. Thus, the invention describedherein is intended to embrace all such alternatives, modifications,applications and variations as may fall within the spirit and scope ofthe appended claims.

What is claimed is:
 1. A laser module comprising: a laser diode having afront facet; an optical fiber having a fiber end disposed to form acoupling region between the front facet and the fiber end to couplelight emitted from the front facet to the optical fiber along a firstpath; and a monitor photo diode disposed to a side of the couplingregion substantially out of the first path to couple light from at leastone of the fiber end and the front facet.
 2. The laser module of claim 1wherein the photo diode is disposed adjacent to the coupling region. 3.The laser module of claim 1 wherein the laser diode has an aperture inthe front facet, the aperture having a fast axis and a slow axis, themonitor photo diode being disposed to couple light from the laser diodein the fast axis.
 4. The laser module of claim 1 wherein the laser diodehas an aperture in the front facet, the aperture having a fast axis anda slow axis, the monitor photo diode being disposed to couple light fromthe laser diode in the slow axis.
 5. The laser module of claim 1 whereinthe monitor photo diode is disposed to couple light reflected from thefiber end.
 6. The laser module of claim 5 further comprising areflectance-increasing coating on the fiber end.
 7. The laser module ofclaim 1 wherein the monitor photo diode is disposed to couple lightemitted from the fiber end.
 8. The laser module of claim 1 wherein thelaser diode and the optical fiber are mechanically coupled to asubstrate and the monitor photo diode is disposed between the couplingregion and the substrate.
 9. The laser module of claim 8 wherein thelaser diode and the optical fiber are mechanically coupled to thesubstrate with a submount.
 10. The laser module of claim 1 wherein theoptical fiber includes a fiber Bragg grating.
 11. The laser module ofclaim 10 wherein the fiber Bragg grating has a reflectivity greater than6% and wherein a first portion of back-reflected light is coupled intothe laser diode and a second portion of back-reflected light is coupledinto the monitor photo diode.
 12. The laser module of claim 1 whereinthe monitor photo diode couples light from the front facet.
 13. Thelaser module of claim 1 wherein the monitor photo diode couples lightfront the front facet and from the fiber end.
 14. The laser module ofclaim 1 wherein the coupling region is on the order of ten microns.